1. Field of the Invention
This invention relates to a screw conveyor type drying apparatus used for drying dehydrated sludge discharged, e.g., from sewage or excrement treatment plants or for drying feed or food which is high in water content.
2. Description of the Prior Art
Japanese Patent Application Laying-Open No. 131976/1982 discloses an example of a screw conveyor type drying apparatus for the aforesaid use, as shown in present FIG. 16 marked "Prior Art". A casing 100 is provided at one end thereof with a charge port 101 for material to be dried and a discharge port 102 for dried material. Disposed within the casing 100 is a hollow drive shaft 103 rotatably supported by the opposite ends of the casing 100. The drive shaft 103 carries a plurality of feed vanes 104 of hollow construction which continuously extended along an imaginary helix positioned on the outer peripheral surface of the drive shaft 103. Further, to dry material, there is provided a mechanism whereby heating fluid 105 is fed into the hollow areas of the feed vanes 104 through the drive shaft 103 and then discharged therefrom.
In the drying apparatus described above, a material to be dried which is charged into the inlet part 101 of the casing 100 is conveyed to the discharge port 102 as the feed vanes 104 are rotated along with the drive shaft 103, and at the same time the material is heated and dried by the heating fluid 105 fed into the feed vanes.
According to the aforesaid drying apparatus, however, since the feed vanes 104 are formed continuously around the periphery of the drive shaft 103, i.e., continuously along the imaginary helix positioned on the outer peripheral surface of the drive shaft 103, there has been a problem that the rate of travel of the material is so high that the material is discharged through the discharge port 102 before it is fully dried. On the other hand, if the rate of travel of the material is reduced, the material cannot be stirred sufficiently. Furthermore, since the material is rotated along with the feed vanes 104, structurally, the stirring efficiency is inherently low; therefore, the material cannot be dried uniformally and the drying efficiency is not sufficiently high.
On the other hand, Japanese Patent Application Laying-Open No. 131976/1982 discloses a conveyor type drying apparatus shown in present FIG. 17 also marked "Prior Art". FIG. 17 shows a hollow drive shaft 113 rotatably installed in a casing 110, with a plurality of hollow vanes 114 attached to the outer periphery of the drive shaft 113. As for the positional arrangement of the feed vanes 114, however, sets of four feed vanes 114 are spaced along the length of the drive shaft 113, the four vanes in each set being spaced around the same circumference. Thus, despite the fact that the feed vanes 114 cross the axis of the drive shaft 113, a material to be dried cannot be dried efficiently. Further, since the four feed vanes 114 in each set are spaced around the same circumference, there has been a problem that the stirring efficiency is not high. As a result, there has been a drawback that conveying and drying a material in such a known apparatus requires a relatively large amount of energy.
Japanese Utility Model Application No. 193994/1984 discloses a conveyor type drying apparatus using paddles 124 which are sector shaped in a plan view as shown in FIGS. 18 and 10, also marked "Prior Art". More particularly, a plurality of pairs of paddles 124 are spaced along the length of a drive shaft 123, the two paddles 124 in each pair being spaced around the same circumference. In this drying apparatus, heating fluid is fed into the paddles 124, whereby a material to be dried which comes in contact with the paddles 124, is dried. As is clear from FIG. 18, the paddles 124 are disposed at right angles to the drive shaft 123. Thus, the paddles 124 themselves do not serve to convey the material; whereby the material cannot be conveyed efficiently. Thus, there has been a drawback that in conveyance, the casing 130 must be inclined or a relatively large amount of energy must be supplied.